Current-perpendicular-to-the-plane (CPP) magnetoresistive sensor with antiparallel-free layer structure and low current-induced noise

ABSTRACT

A current-perpendicular-to-the-plane (CPP) magnetoresistive sensor has an antiparallel free (APF) structure as the free layer and a specific direction for the applied bias or sense current. The (APF) structure has a first free ferromagnetic (FL 1 ), a second free ferromagnetic layer (FL 2 ), and an antiparallel (AP) coupling (APC) layer that couples FL 1  and FL 2  together antiferromagnetically with the result that FL 1  and FL 2  have substantially antiparallel magnetization directions and rotate together in the presence of a magnetic field. The thickness of FL 1  is preferably greater than the spin-diffusion length of the electrons in the FL 1  material. The minimum thickness for FL 2  is a thickness resulting in a FL 2  magnetic moment equivalent to at least 10 Å Ni 80 Fe 20  and preferably to at least 15 Å Ni 80 Fe 20 . The CPP sensor operates specifically with the conventional sense current (opposite the electron current) directed from the pinned ferromagnetic layer to the APF structure, which results in suppression of current-induced noise.

RELATED APPLICATION

This application is a continuation-in-part of application Ser. No. 11/380,625 filed Apr. 27, 2006, now U.S. Pat. No. 7,580,229.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a CPP sensor with low current-induced noise.

2. Background of the Invention

One type of conventional magnetoresistive sensor used as the read head in magnetic recording disk drives is a “spin-valve” (SV) sensor. A SV magnetoresistive sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer has its magnetization direction “free” to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance.

In a magnetic recording disk drive SV read sensor or head, the stack of layers are located in the read “gap” between magnetic shields. The magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance. If the sense current flowing through the SV is directed parallel to the planes of the layers in the sensor stack, the sensor is referred to as a current-in-the-plane (CIP) sensor, while if the sense current is directed perpendicular to the planes of the layers in the sensor stack, it is referred to as current-perpendicular-to-the-plane (CPP) sensor.

CPP-SV read heads are described by A. Tanaka et al., “Spin-valve heads in the current-perpendicular-to-plane mode for ultrahigh-density recording”, IEEE TRANSACTIONS ON MAGNETICS, 38 (1): 84-88 Part 1 January 2002. Another type of CPP sensor is a magnetic tunnel junction (MTJ) sensor in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a MTJ magnetoresistive read head the spacer layer is electrically insulating and is typically alumina (Al₂O₃ ) or MgO; in a CPP-SV magnetoresistive read head the spacer layer is electrically conductive and is typically copper.

CPP sensors are susceptible to current-induced noise and instability. The spin-polarized current flows perpendicularly through the ferromagnetic layers and produces a spin transfer torque on the local magnetization. This can produce continuous gyrations of the magnetization, resulting in substantial low-frequency magnetic noise if the sense current is above a certain level. This effect is described by J.-G. Zhu et al., “Spin transfer induced noise in CPP read heads,” IEEE TRANSACTIONS ON MAGNETICS, Vol. 40, pp. 182-188, January 2004. In a related paper it was suggested, but not demonstrated, that the sensitivity to spin-torque-induced instability of the free layer could be reduced by use of a dual spin-valve. (J.-G. Zhu et al., “Current induced noise in CPP spin valves,” IEEE TRANSACTIONS ON MAGNETICS, Vol. 40, No. 4, pp. 2323-2325, July 2004). However, a dual spin-valve requires a second spacer layer on the free layer and a second pinned layer on the second spacer layer, and thus results in a larger shield-to-shield read gap distance, which lowers sensor resolution.

What is needed is a CPP sensor that produces minimal current-induced noise without loss of magnetoresistance or sensor resolution.

SUMMARY OF THE INVENTION

The invention is a CPP magnetoresistive sensor with an antiparallel free (APF) structure and a specific direction for the applied bias or sense current that result in both increased magnetoresistance and, more importantly, reduced susceptibility to current-induced instability and noise. The (APF) structure has a first free ferromagnetic (FL1) adjacent the CPP sensor's nonmagnetic spacer layer, a second free ferromagnetic layer (FL2), and an antiparallel (AP) coupling (APC) layer that couples FL1 and FL2 together antiferromagnetically with the result that FL1 and FL2 have substantially antiparallel magnetization directions. The sensor may also include a ferromagnetic biasing layer formed from CoPt or some other hard magnetic material outside of, and electrically insulated from, the sensor stack near the side edges of the free ferromagnetic layer (typically FL1), with sufficiently large magnetic moment to longitudinally bias the net magnetization of the APF structure. The antiferromagnetically-coupled FL1 and FL2 rotate together in the presence of a magnetic field, such as the magnetic fields from data recorded in a magnetic recording medium. The thicknesses of FL1 and FL2 are chosen to obtain the desired net free layer magnetic moment/area for the sensor, which for advanced CPP read heads is no greater than the equivalent moment/area of approximately 100 Å of Ni₈₀Fe₂₀. The sensor is based on the discovery that for a CPP sensor with an appropriate APF structure, the critical current above which current-induced noise occurs is substantially higher when the applied sense current is directed from the pinned layer to the APF structure. Thus the sensor operates with the sense current I_(S) (which is the conventional current and opposite in direction to the electron current I_(e)) directed from the reference ferromagnetic layer to the APF structure, so that the spin-torque effect, and thus current-induced noise, is suppressed. This allows much higher sense currents to be used, resulting in higher sensor output voltage.

Also, to maximize the sensor signal, the thickness of FL1 can be chosen to be greater than the spin-diffusion length of electrons in the FL1 material, which maximizes the bulk spin-dependent scattering in FL1. It is also known, through both direct experimentation and modeling, that the critical current that results in free-layer instability can be substantially increased with increasing thickness of FL2, so that it would appear to be desirable to maximize the thickness of FL2 or optimize it relative to the thickness of FL1. However, because the magnetization of FL2 is antiparallel to the magnetization of the biasing layer in the absence of an external magnetic field, if the magnetic moment of FL2 is too high, which can occur if FL2 is too thick, the APF structure may become magnetostatically unstable (at any bias current).

In this invention it has also been discovered, through experiment and modeling, that an FL2 thickness resulting in a magnetic moment equivalent to about 15 Å Ni₈₀Fe₂₀ gives a substantial increase in critical current and that the absolute value of the critical current is relatively independent of FL1 thickness when FL2 is fixed at this equivalent thickness. Thus, although the free-layer critical current for the APF structure may be increased even further with thicker FL2, such a thicker FL2 will likely offer little to no spin-torque stability advantage for the read head, which will instead likely be limited instead by the reference layer critical current. Rather a thicker FL2 will quickly weaken magnetostatic stability. Thus, keeping FL2 no thicker than that needed to provide enough margin in critical current so that spin-torque-induced free-layer instability is not the limiting factor on read head performance, will also minimize magnetostatic instability of the FL1/FL2 couple.

For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic top view of a conventional magnetic recording hard disk drive with the cover removed.

FIG. 2 is an enlarged end view of the slider and a section of the disk taken in the direction 2-2 in FIG. 1.

FIG. 3 is a view in the direction 3-3 of FIG. 2 and shows the ends of the read/write head as viewed from the disk.

FIG. 4 is a cross-sectional schematic view of a current-perpendicular-to-the-plane spin-valve (CPP-SV) read head showing the stack of layers located between the magnetic shield layers.

FIG. 5 is a cross-sectional schematic view of a CPP-SV read head according to this invention.

FIG. 6 is a graph of noise power spectral density (PSD) as a function of electron current I_(e) at positive and negative applied magnetic fields for a control sample according to the prior art with a single free layer, wherein positive electron current I_(e) corresponds to electron flow from the pinned layers towards the free layers.

FIG. 7 is a graph of noise power spectral density (PSD) as a function of electron current I_(e) at positive and negative applied magnetic fields for a sample according to this invention with an antiparallel-free structure, wherein positive electron current I_(e) corresponds to electron flow from the pinned layers towards the free layers.

FIG. 8 is a graph of positive and negative critical electron currents I_(e) ^(crit) for spin-torque-induced noise as a function of the second free ferromagnetic layer (FL2) thickness.

FIG. 9 is a numerical simulation of the negative critical current density (for the parallel state) for an antiparallel free (APF) structure as a function of the first free ferromagnetic layer (FL1) thickness for four values of FL2 thickness.

DETAILED DESCRIPTION OF THE INVENTION

The CPP-SV read head has application for use in a magnetic recording disk drive, the operation of which will be briefly described with reference to FIGS. 1-3. FIG. 1 is a block diagram of a conventional magnetic recording hard disk drive. The disk drive includes a magnetic recording disk 12 and a rotary voice coil motor (VCM) actuator 14 supported on a disk drive housing or base 16. The disk 12 has a center of rotation 13 and is rotated in direction 15 by a spindle motor (not shown) mounted to base 16. The actuator 14 pivots about axis 17 and includes a rigid actuator arm 18. A generally flexible suspension 20 includes a flexure element 23 and is attached to the end of arm 18. A head carrier or air-bearing slider 22 is attached to the flexure 23. A magnetic recording read/write head 24 is formed on the trailing surface 25 of slider 22. The flexure 23 and suspension 20 enable the slider to “pitch” and “roll” on an air-bearing generated by the rotating disk 12. Typically, there are multiple disks stacked on a hub that is rotated by the spindle motor, with a separate slider and read/write head associated with each disk surface.

FIG. 2 is an enlarged end view of the slider 22 and a section of the disk 12 taken in the direction 2-2 in FIG. 1. The slider 22 is attached to flexure 23 and has an air-bearing surface (ABS) 27 facing the disk 12 and a trailing surface 25 generally perpendicular to the ABS. The ABS 27 causes the airflow from the rotating disk 12 to generate a bearing of air that supports the slider 22 in very close proximity to or near contact with the surface of disk 12. The read/write head 24 is formed on the trailing surface 25 and is connected to the disk drive read/write electronics by electrical connection to terminal pads 29 on the trailing surface 25.

FIG. 3 is a view in the direction 3-3 of FIG. 2 and shows the ends of read/write head 24 as viewed from the disk 12. The read/write head 24 is a series of thin films deposited and lithographically patterned on the trailing surface 25 of slider 22. The write head includes magnetic write poles P1/S2 and P1 separated by a write gap 30. The CPP-SV magnetoresistive sensor or read head 100 is located between two magnetic shields S1 and P1/S2, with P1/S2 also serving as the first write pole for the write head. The shields S1, S2 are formed of magnetically permeable material and are electrically conductive so they can function as the electrical leads to the read head 100. Separate electrical leads may also be used, in which case the read head 100 is formed in contact with layers of electrically conducting lead material, such as tantalum, gold, or copper, that are in contact with the shields S1, S2.

FIG. 4 is an enlarged sectional view showing the layers making up sensor 100 as seen from the air bearing surface (ABS) of the sensor. Sensor 100 is a CPP-SV read head comprising a stack of layers formed between the two magnetic shield layers S1, S2 that are typically electroplated NiFe alloy films. The lower shield S1 is typically polished by chemical-mechanical polishing (CMP) to provide a smooth substrate for the growth of the sensor stack. This may leave an oxide coating which can be removed with a mild etch just prior to sensor deposition. The sensor layers include a pinned ferromagnetic layer 120 having a fixed magnetic moment or magnetization direction 121 oriented substantially perpendicular to the ABS (into the page), a free ferromagnetic layer 110 having a magnetic moment or magnetization direction 111 that is oriented substantially parallel to the ABS in the absence of an external magnetic field and that can rotate in the plane of layer 110 in response to transverse external magnetic fields from the disk 12, and an electrically conducting spacer layer 130, typically copper (Cu), between the pinned layer 120 and the free layer 110.

The pinned ferromagnetic layer in a CPP-SV sensor may be a single pinned layer or an antiparallel (AP) pinned structure. An AP-pinned structure has first (AP1) and second (AP2) ferromagnetic layers separated by a nonmagnetic antiparallel coupling (APC) layer with the magnetization directions of the two AP-pinned ferromagnetic layers oriented substantially antiparallel. The AP2 layer, which is in contact with the nonmagnetic APC layer on one side and the sensor's electrically conducting spacer layer on the other side, is typically referred to as the reference layer. The AP1 layer, which is typically in contact with an antiferromagnetic or hard magnet pinning layer on one side and the nonmagnetic APC layer on the other side, is typically referred to as the pinned layer. The AP-pinned structure minimizes the net magnetostatic coupling between the reference/pinned layers and the CPP-SV free ferromagnetic layer. The AP-pinned structure, also called a “laminated” pinned layer, and sometimes called a synthetic antiferromagnet (SAF), is described in U.S. Pat. No. 5,465,185.

The pinned layer in the CPP-SV sensor in FIG. 4 is an AP-pinned structure with reference ferromagnetic layer 120 (AP2) and a lower ferromagnetic layer 122 (AP1) that are antiferromagnetically coupled across an AP coupling (APC) layer 123. The APC layer 123 is typically Ru, Ir, Rh, Cr or alloys thereof. The free ferromagnetic layer 110, spacer layer 130 and AP2 layer 120 together make up what is call the “active region” of the sensor. The AP1 and AP2 ferromagnetic layers have their respective magnetization directions 127, 121 oriented antiparallel. The API layer 122 may have its magnetization direction pinned by being exchange-coupled to an antiferromagnetic (AF) layer 124 as shown in FIG. 4. Alternatively, the AP-pinned structure may be “self-pinned” or it may be pinned by a hard magnetic layer such as Co_(100-x)Pt_(x) or Co_(100-x-y)Pt_(x)Cr_(y) (where x is about between 8 and 30 atomic percent). In a “self pinned” sensor the AP1 and AP2 layer magnetization directions 127, 121 are typically set generally perpendicular to the disk surface by magnetostriction and the residual stress that exists within the fabricated sensor. It is desirable that the AP1 and AP2 layers have similar moments. This assures that the net magnetic moment of the AP-pinned structure is small so that magneto-static coupling to the free layer is minimized and the effective pinning field of the AF layer 124, which is approximately inversely proportional to the net magnetization of the AP-pinned structure, remains high. In the case of a hard magnet pinning layer, the hard magnet pinning layer moment needs to be accounted for when balancing the moments of AP1 and AP2 to minimize magnetostatic coupling to the free layer.

Located between the lower shield layer S1 and the AP-pinned structure are the bottom electrical lead 126 and a seed layer 125. The seed layer 125 may be a single layer or multiple layers of different materials. Located between the free ferromagnetic layer 110 and the upper shield layer S2 are a capping layer 112 and the top electrical lead 113. The capping layer 112 may be a single layer or multiple layers of different materials, such as a Cu/Ru/Ta trilayer.

In the presence of an external magnetic field in the range of interest, i.e., magnetic fields from recorded data on the disk 12, the magnetization direction 111 of free layer 110 will rotate while the magnetization direction 121 of reference layer 120 will remain substantially fixed and not rotate. The rotation of the free-layer magnetization 111 relative to the reference-layer magnetization 121 results in a change in electrical resistance. Hence, when a sense current I_(S) is applied between top lead 113 and bottom lead 126, the resistance change is detected as a voltage signal proportional to the strength of the magnetic signal fields from the recorded data on the disk.

The leads 126, 113 are typically Ta or Rh. However, any low resistance material may also be used. They are optional and used to adjust the shield-to-shield spacing. If the leads 126 and 113 are not present, the bottom and top shields S1 and S2 are used as leads. The seed layer 125 is typically one or more layers of NiFeCr, NiFe, Ta, Cu or Ru. The AF layer 124 is typically a Mn alloy, e.g., PtMn, NiMn, FeMn, IrMn, IrMnCr, PdMn, PtPdMn or RhMn. If a hard magnetic layer is used instead of an AF layer it is typically a CoPt or FePt alloy, for example CoPtCr. The capping layer 112 provides corrosion protection and is typically formed of Ru or Ta.

The ferromagnetic layers 122 (AP1), 120 (AP2), and 110 (free layer) are typically formed of crystalline CoFe or NiFe alloys, or a multilayer of these materials, such as a CoFe/NiFe bilayer. These alloys have a sufficiently high magnetic moment M and bulk electron scattering parameter β, but a relatively low electrical resistivity ρ. The AP2 layer can also be a laminated structure to obtain a high degree of spin-dependent interface scattering. For example the AP2 layer can be a FM/XX/FM/ . . . /XX/FM laminate, where the ferromagnetic (FM) layers are formed of Co, Fe or Ni, one of their alloys, or a multilayer of these materials, such as a CoFe—NiFe—CoFe trilayer; and the XX layers are nonmagnetic layers, typically Cu, Ag, or Au or their alloys, and are thin enough that the adjacent FM layers are strongly ferromagnetically coupled.

For example, AP2 layer 120 may be a CoFe alloy, typically 10 to 30 Å thick, and the free ferromagnetic layer 110 may be a bilayer of a CoFe alloy, typically 10-15 Å thick and formed on the spacer layer 130, with a NiFe alloy, typically 10-30 Å thick, formed on the CoFe layer of the bilayer. The APC layer in the AP-pinned structure is typically Ru or Ir with a thickness between about 4-10 Å.

If the AP-pinned structure is the “self-pinned” type, then no antiferromagnetic pinning layer is required. In a self-pinned structure where no antiferromagnet or hard magnet pinning layer is present, the AP1 layer is in contact with a seed layer on the sensor substrate.

A ferromagnetic biasing layer 115, such as a CoPt or CoCrPt hard magnetic bias layer, is also typically formed outside of the sensor stack near the side edges of the free ferromagnetic layer 110. The biasing layer 115 is electrically insulated from free layer 110 by insulating regions 116, which may be formed of alumina, for example. The biasing layer 115 has a magnetization 117 generally parallel to the ABS and thus longitudinally biases the magnetization 111 of the free layer 110. Hence in the absence of an external magnetic field its magnetization 117 is parallel to the magnetization 111 of the free layer 110. The ferromagnetic biasing layer 115 may be a hard magnetic bias layer or a ferromagnetic layer that is exchange-coupled to an antiferromagnetic layer.

One or more of the free layer 110, the AP2 layer 120, the capping layer 112 and the conductive nonmagnetic spacer layer 130 may also include a nano-oxide layer (NOL) to locally confine the current path and increase the effective resistance of the active region. A CoFe NOL may be formed, for example, by interrupting the deposition after some CoFe has been deposited somewhere in the free layer, the AP2 layer, the capping layer, or the conductive spacer layer and oxidizing its surface for several minutes in an O₂ or O₂/Ar gas at 0.1-10 Torr. NOLs can be formed by oxidizing other materials, e.g., Cu/Al or Cu/Ti alloys or multilayers.

While the read head 100 shown in FIG. 4 is a “bottom-pinned” read head because the AP-pinned structure is below the free layer 110, the free layer 110 can be located below the AP-pinned structure. In such an arrangement the layers of the AP-pinned structure are reversed, with the AP2 layer 120 on top of and in contact with the spacer layer 130.

In this invention the CPP sensor is similar to the CPP sensor described above, but has an antiparallel-free (APF) structure as the free layer, and a specific direction for the applied bias or sense current, that result in both increased magnetoresistance and reduced susceptibility to current-induced instability and noise. While the invention will be described with respect to a CPP-SV read head, the invention is applicable to other types of CPP magnetoresistive sensors such as MTJ sensors, and to CPP sensors with NOLs.

The CPP-SV sensor of this invention is illustrated in FIG. 5 as sensor 200. The free layer is an antiparallel free (APF) structure comprising a first free ferromagnetic layer 201 (FL1), second free ferromagnetic layer 202 (FL2), and an antiparallel (AP) coupling (APC) layer 203. APC layer 203, such as a thin (between about 4 Å and 10 Å) Ru film, couples FL1 and FL2 together antiferromagnetically with the result that FL1 and FL2 maintain substantially antiparallel magnetization directions, as shown by arrows 211, 212, respectively. The antiferromagnetically-coupled FL1 and FL2 rotate together in the presence of a magnetic field, such as the magnetic fields from data recorded in a magnetic recording medium. The net magnetic moment/area of the APF structure (represented by the difference in magnitudes of arrows 211, 212) is (M1*t1−M2*t2), where M1 and t1 are the saturation magnetization and thickness, respectively, of FL1, and M2 and t2 are the saturation magnetization and thickness, respectively, of FL2. Thus the thicknesses of FL1 and FL2 are chosen to obtain the desired net free layer magnetic moment for the sensor. In advanced CPP read heads, it is desirable that the net magnetic moment/area of the free layer (M1*t1−M2*t2), be no greater than the equivalent moment/area of approximately 100 Å of NiFe. As used herein in reference to equivalent moments and equivalent thicknesses, NiFe shall mean a Ni₈₀Fe₂₀ alloy, i.e. a NiFe alloy with 20 atomic percent Fe.

In FIG. 5, the free ferromagnetic layer 201 (FL1) is shown as having the greater magnetic moment, as represented by the greater length of arrow 211. Thus a ferromagnetic biasing layer 215, which may be a hard magnetic layer such as a CoPt or CoCrPt layer, is formed outside of the sensor stack near the side edges of the free ferromagnetic layer 201 (FL1) and has a magnetization 217 that is parallel to the ABS. The biasing layer 215 is electrically insulated from FL1 by insulating regions 216, which may be formed of alumina, for example. Thus biasing layer 215 longitudinally biases the net magnetization of the APF structure and its magnetization 217 is parallel to the magnetization 211 of FL1 in the absence of an external magnetic field. However, as shown in FIG. 5, the magnetization 217 of biasing layer 215 is antiparallel to the magnetization 212 of FL2 layer 202 in the absence of an external magnetic field. As a result, if the magnetic moment of FL2 is too high, which can occur if FL2 is too thick, the APF structure may be magnetostatically unstable. Thus while the thicknesses of FL1 and FL2 can be chosen to obtain a desired net free layer magnetic moment for the sensor, it is at the same time desirable to keep FL2 as thin as possible, not only for magnetostatic stability, but to also minimize the overall thickness of the sensor stack.

An APF structure as a free layer in a CIP sensor was first described in U.S. Pat. No. 5,408,377. A specific type of Fe-based APF structure as a free layer in a CPP sensor is described in US 2005/0243477 A1, assigned to the same as assignee as this application. In the sensor described in that application the objective is high magnetoresistance, so the free layer material near the spacer layer is Fe and the spacer layer is thus required to be Cr because Fe/Cr multilayers are a well-known system exhibiting giant magnetoresistance. The use of pure Fe in the free layer near the spacer layer requires that a second free layer farther from the spacer layer be used to lower the coercivity of the Fe-based APF and that the second free layer be alloyed with nitrogen (N) to further reduce the coercivity. However, it is not preferred to use pure elements such as Fe in the free layer or any other active magnetic layer of a CPP sensor. Rather, it is preferred to use an alloy, such as a FeCo alloy, to increase electron scattering and thus to shorten the spin-diffusion length. This is important to achieve high magnetoresistance with thin magnetic layers, resulting in a thin sensor capable of high resolution. Also, pure Fe is corrosive and should be avoided for reliability reasons.

In this invention the spacer layer 230 is not formed of Cr, but is preferably Cu, Au or Ag which yield higher CPP magnetoresistance in contact with Co_(x)Fe_(1-x) (50<x<100 at. %) and Ni_(x)Fe_(1-x) (50<x<100 at. %) magnetic layers. Also, in this invention FL1 and FL2 are not formed of Fe or FeN due to their susceptibility to corrosion at the air-bearing surface, but may be formed of crystalline CoFe or NiFe alloy, or a multilayer of these materials, such as a CoFe/NiFe bilayer. FL1 and FL2 may also be formed of relatively high-resistance amorphous alloys, such as an alloy of one or more elements selected from Co, Fe and Ni, and at least one nonmagnetic element that is present in an amount that renders the otherwise crystalline alloy amorphous. Examples of amorphous alloys for FL1 and FL2 include Co_((100-x-y))Fe_(x)X_(y) and Ni_((100-x-y))Fe_(x)X_(y) where X is B, Si or Tb, and y is between about 5 and 40 atomic percent (at .%). FL1 and FL2 may also be formed of a ferromagnetic Heusler alloy, i.e., a metallic compound having a Heusler alloy crystal structure. Examples of Heusler alloys for FL1 and FL2 include Co₂MnX (where X is Al, Sb, Si or Ge), NiMnSb, PtMnSb, and Co₂Fe_(x)Cr_((1-x))Al (where x is between 0 and 1). FL1 and FL2 may also be formed of a high-resistance ferromagnetic crystalline alloy based on CoFe or NiFe alloys, with short spin diffusion length (<100 Å) and resulting in appreciable CPP magnetoresistance, and containing one or more additions of Cu, Mg, Al, Si, Au, Ag or B. Also, in this invention the APC layer 203 is not formed of Cr, but is preferably Ru, Ir, or Rh or alloys thereof.

In this invention, it can be advantageous to select the composition and thickness of FL1 to be greater than the spin-diffusion length of the electrons in the FL1 material to maximize the bulk spin-dependent scattering of electrons and thus maximize the sensor signal. If the FL1 thickness is significantly less than the spin-diffusion length, (which is about 40 Å for NiFe), then the signal due to FL1 is not maximized. In addition, FL2 may participate in the spin-valve effect and may, due to its magnetization being opposite in direction to that of FL1, cause a negative contribution to the magnetoresistance. If FL1 is sufficiently thick (substantially above the spin-diffusion length or at least about 110% of the spin-diffusion length), then the signal from FL1 is close to maximum, and there is no appreciable negative spin-valve contribution from FL2. The spin-diffusion length depends on the magnetic material and is about 40 Å for NiFe, 500 Å for Co, and 120 Å for Co₉₀Fe₁₀. If materials that have a high spin-diffusion length are used for FL1 then the APF structure can be made thicker if an optimum signal amplitude is desired, but that will increase the overall shield-to-shield spacing of the sensor. The spin-diffusion length in any material may be determined through a series of experiments measuring the dependence of the CPP spin-valve effect on the thickness of the material of interest. The material of interest may be included either in the spacer layer or be substituted for one of the ferromagnetic layers. See for example A. C. Reilly et al., J. Mag. Mag. Mat. 195, L269-L274 (1999). These experiments are somewhat difficult and tedious, so the spin-diffusion length is not yet known for a large number of materials. One general trend is that the spin-diffusion length varies inversely with the electrical resistivity, i.e., materials with a large electrical resistivity display a short spin-diffusion length. In general, alloys exhibit higher resistivity than pure metals, i.e, unalloyed elemental metals, and thus shorter spin-diffusion lengths. Thus, in general, alloys tend to have shorter spin diffusion lengths than pure metals because they exhibit a larger resistivity than the elements they are comprised of due to enhanced electron scattering.

In this invention, as shown in FIG. 5, the direction of the sense current I_(S) (which is the conventional current and opposite in direction to the electron current) from current source 150 is specifically chosen to be from the AP-pinned structure to the APF structure (from bottom lead 126 to top lead 113), so that possible spin-torque induced instability is suppressed, as will be explained below. To achieve this reduction in spin-torque instability, or concomitantly, increase in the critical current for instability onset, it has been determined that FL2 must be sufficiently thick. At the same time, as explained above, if FL2 is too thick the APF structure may be magnetostatically unstable as a result of the magnetization of F2 being antiparallel to the magnetization of the ferromagnetic biasing layer. Thus, in this invention the thickness of FL2 is chosen to be as thin as possible, but thick enough to assure that the spin-torque induced instability of the free-layer (or APF structure) is not the limiting factor in the performance of the read head. From experimental data described below FL2 can be as thin as approximately 10 Å of Ni₈₀Fe₂₀ equivalent moment, and still provide sufficient immunity from spin-torque instability.

CPP test samples with APF structures of this invention (similar to FIG. 5) were compared with a CPP control sample with a single free layer (similar to FIG. 4). The material for the free layers was a NiFe alloy with 2 at. % Tb (Ni₈₃Fe₁₅Tb₂), where the subscripts represent atomic percent. The control had a free layer thickness of 40 Å, a ΔRA of about 0.75 mOhm-μm², and a ΔR/R of about 2.35%. In the CPP test sample if the APF structure had FL1=50 Å and FL2=10 Å there was no significant difference in ΔRA from the control. This is likely because the positive effect of the thicker FL1 (50 Å compared to the 40 Å control) is compensated by the negative effect of the FL2 layer. However, if FL2 is thicker (FL2=20 Å and FL1=60 Å; or FL2=30 Å and FL1=70 Å), then the ΔRA signal is higher than the control (0.87 mOhm-μm² and 0.90 mOhm-μm² for FL1=60 Å and 70 Å, respectively). Although FL2 increased in thickness, it did not result in any noticeable decrease in ΔRA because FL1 is substantially greater than the spin-diffusion length in NiFe or NiFeTb (which is at most about 40-50 Å), and the Ru APC layer (layer 203 in FIG. 5) further mixes the electron spins, so that FL2 becomes “disconnected” from the FL1 reference layer (layer 120 in FIG. 5) and cannot significantly participate in the spin-valve effect. For the test sample with FL1=50 Å and FL2=10 Å the ΔR/R decreased from the control value of 2.35% to 2.15%, but for the test sample with FL1=60 Å and FL2=20 Å the ΔR/R then increased to about 2.5%. As FL1 is made even thicker (keeping the FL2−FL1 thickness difference at about 40 Å) it is expected from the considerations above that ΔRA would saturate to a nearly constant value, while ΔR/R would also saturate but eventually decrease as RA increases with additional layer thickness. Thus CPP sensors with thicker FL1 layers (FL1=60 Å or FL1=70 Å) are optimized for a magnetoresistive response.

In another set of test samples, the APF layer structure comprised a trilayer FL1=CoFe(6 Å)/NiFe(t1)/CoFe(2 Å) and a bilayer FL2=CoFe(2 Å)/NiFe(t2), and the thicknesses t1 and t2 were chosen so that the net moment/area (M1*t1−M2*t2) of the APF structure was constant at the equivalent moment/area of about 45 Å of NiFe. The control sample with a simple free layer (no FL2, and FL1=6 Å CoFe/38 Å NiFe/2 Å CoFe) had a ΔR/R of about 2.13%. For the test samples with an APF structure and t2 values from 5 Å to 45 Å (t1 values from 45 Å to 85 Å) the ΔR/R increased to about 2.7% to 2.8%. The ΔRA increased from 0.68 mOhm-μm² for the sample with the simple free layer structure to 0.93 mOhm-μm² for the sample with the APF structure with t2=15 Å of NiFe.

An important feature of this invention, in addition to an improved magnetoresistance, is that the CPP sensor exhibits substantial immunity to current-induced noise when the bias or sense current I_(S) is in the direction from the pinned layer to the APF structure (from lead 126 to lead 113 in FIG. 5). For the purpose of FIGS. 6-7, this actual sense current direction corresponds to “negative” electron current I_(e) (i.e., the electrons flow from the APF structure to the pinned layer).

FIG. 6 shows the noise power spectral density (PSD) for a control sample with a single free layer (FL2=none; t1=38 Å NiFe). Positive applied magnetic fields (curve 300) correspond to the pinned and free layer net magnetizations being antiparallel, and negative applied fields (curve 310) correspond to the pinned and free layer net magnetizations being parallel. It can be seen that for a “positive” electron current I_(e) (actual sense current I_(S) from the free layer to the pinned layer or from top to bottom in FIG. 4) the antiparallel curve 300 shows a large increase in noise power above a “critical current” of about 0.6 mA, while for negative electron currents the noise increase for the parallel orientation (curve 310) is above critical current of about 1.5 mA. This behavior is expected in the present understanding of current-induced noise in spin-valve structures. By comparison with FIG. 6, FIG. 7 shows the PSD for a sample with an APF structure like this invention, with t1=65 Å NiFe and t2=25 Å NiFe. Positive applied magnetic fields (curve 400) correspond to the pinned and free layer net magnetizations being antiparallel, and negative applied fields (curve 410) correspond to the pinned and free layer net magnetizations being parallel.

A comparison of FIGS. 6 and 7 shows that while the behavior for positive electron currents is qualitatively similar (i.e., a large increase in noise power above a critical current of about 0.6 mA), the behavior for negative electron currents is unexpected and unique. In particular, in FIG. 7 the noise power remains extremely low for both parallel and antiparallel orientations, up to large values of the electron current (“more negative” than a critical current of about 6.5 mA, although the exact value is not known due to limitations in the measurement method).

FIG. 8 is a graph of positive and negative critical electron currents for spin-torque-induced noise as a function of the FL2 thickness. (Negative electron current corresponds to electrons flowing from the APF structure to the pinned layer, which corresponds to bias or sense current from the pinned layer to the APF structure). As shown in FIG. 8, the negative critical current increases (becomes more negative) with increasing FL2 thickness. Sensors with FL2 equal to and greater than about 15 Å NiFe will exhibit high negative critical currents for spin-torque instability. Thus, there is a range of APF structures (with appropriate FL1 and FL2 equivalent NiFe thicknesses) which display this spin-torque immunity for negative electron current (bias or sense current I_(S) in the direction from the pinned layer to the APF structure), in addition to an improved magnetoresistive response.

Thus FIG. 8 shows that FL2 must have some minimum thickness to contribute to the sensor's immunity to spin-torque instability for negative electron current. However, as described above, it is also desirable to keep FL2 as thin as possible to assure magnetostatic stability of the APF structure as well as to minimize the overall thickness of the sensor stack.

FIG. 9 is a numerical simulation of the negative critical current density (for the parallel state of APF structure and pinned layers) for an APF structure, as a function of the FL1 thickness (equivalent NiFe thickness) for four values (0, 5 Å, 10 Å, and 15 Å) of FL2 thickness. The values are similar to that described in N. Smith et al., “Coresonant Enhancement of Spin-Torque Critical Currents in Spin Valves with a Synthetic-Ferrimagnet Free Layer”, PHYSICAL REVIEW LETTERS, 101, 247205 (2008). FIG. 9 demonstrates that the fractional (or percentage) increase in negative critical current relative to the FL2=0 control case is greater for larger values of FL2/FL1. However, more importantly, it also demonstrates that maintaining an absolute value of FL2=15 Å NiFe is sufficient to give a substantial, i.e., greater than three-fold, increase in negative critical current (relative to the FL2=0 control) even for FL1 thicknesses up to 100 Å NiFe, and that the absolute value of the negative critical current for the APF structure is relatively independent of FL1 thickness when FL2 is fixed at a thickness resulting in a magnetic moment equivalent to 15 Å NiFe. Practical experience has shown that a three-fold increase in critical current is more than sufficient margin such that spin-torque-induced instability of the free-layer is not the limiting factor on read head performance. Thus, although the critical current for the APF structure would be predicted to quickly increase even further with thicker FL2, such a thicker FL2 will offer little to no spin-torque stability advantage for the read head, but will quickly become costly in weakening magnetostatic stability.

While an FL2 with a thickness no greater than that resulting in a magnetic moment equivalent to 15 Å NiFe provides a sufficient critical current and is thus the preferred or optimum minimum thickness, FIG. 9 shows that a FL2 with a thickness no greater than that resulting in a magnetic moment equivalent to 10 Å NiFe will still result in a significant increase in negative critical current. Thus in the present invention the minimum thickness for FL2 is a thickness resulting in a FL2 magnetic moment equivalent to approximately 10 Å NiFe and preferably to approximately 15 Å NiFe. Thus a CPP sensor with an APF structure and sense current applied in a direction from the pinned layer to the APF structure can be achieved if FL2 has a thickness equal to or greater than a thickness that results in a FL2 magnetic moment equivalent to 10 Å Ni₈₀Fe₂₀ and equal to or less than a thickness that results in a FL2 magnetic moment equivalent to 15 Å Ni₈₀Fe₂₀.

Thus the CPP sensor according to this invention allows a much larger bias or sense current to be applied before current-induced noise occurs, provided the sense current I_(S) is applied in the direction from the pinned layer to the APF structure (electron current I_(e) from the APF structure to the pinned layer). The increase in critical current for current-induced noise by a factor of three or more can provide a corresponding increase in output voltage for the sensor.

In this invention the effect on sensor operation due to the increase in critical current (by a factor of three or more) is potentially much larger than the increase in output voltage due to the increase in magnetoresistance (by 10-30%) from the APF structure. Consequently, if the thinnest possible sensor (or a much smaller free layer magnetization) is desired, the increase in critical current, and corresponding increase in sensor output voltage, can still be realized using thinner FL1 and FL2 layers in the APF structure (and correspondingly smaller magnetoresistance).

While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims. 

1. A magnetoresistive sensor having electrical leads for connection to a source of sense current and comprising: one and only one pinned ferromagnetic layer having an in-plane magnetization direction; an antiparallel free (APF) structure comprising a first free ferromagnetic layer (FL1) having an in-plane magnetization, a second free ferromagnetic layer (FL2) having an in-plane magnetization substantially antiparallel to the magnetization of FL1 and a magnetic moment less than that of FL1, and an AP coupling (APC) layer between and in contact with FL1 and FL2 and consisting essentially of a material selected from the group consisting of Ru, Ir, Rh and alloys thereof, wherein FL2 has a thickness equal to or greater than a thickness that results in a FL2 magnetic moment equivalent to 10 Å Ni₈₀Fe₂₀; and an electrically conductive spacer layer between the pinned ferromagnetic layer and FL1 and consisting essentially of an element selected from the group consisting of Cu, Ag and Au; and wherein the sensor is capable of sensing external magnetic fields when a sense current is applied perpendicular to the planes of the layers in the sensor in a direction from the pinned ferromagnetic layer to the APF structure.
 2. The sensor of claim 1 wherein FL2 has a thickness equal to or greater than a thickness that results in a FL2 magnetic moment equivalent to 15 Å Ni₈₀Fe₂₀.
 3. The sensor of claim 1 wherein FL2 has a thickness equal to or greater than a thickness that results in a FL2 magnetic moment equivalent to 10 Å Ni₈₀Fe₂₀ and equal to or less than a thickness that results in a FL2 magnetic moment equivalent to 15 Å Ni₈₀Fe₂₀.
 4. The sensor of claim 1 further comprising a ferromagnetic biasing layer for biasing the magnetization of the APF structure, the biasing layer having a magnetization substantially antiparallel to the magnetization of FL2 in the absence of an external magnetic field.
 5. The sensor of claim 4 wherein the biasing layer is a hard magnetic layer.
 6. The sensor of claim 1 further comprising a substrate, a first electrical lead layer on the substrate, and a second electrical lead layer.
 7. The sensor of claim 6 wherein the pinned ferromagnetic layer is on the first lead layer, the spacer layer is on the pinned ferromagnetic layer, FL1 is on the spacer layer, the second lead layer is on the APF structure, and the direction of sense current to be applied to the sensor is in the direction from the first lead layer to the second lead layer.
 8. The sensor of claim 1 wherein FL1 comprises a crystalline alloy of Fe and at least one of Ni and Co.
 9. The sensor of claim 1 wherein FL1 comprises an amorphous ferromagnetic alloy comprising a nonmagnetic element X and at least one of Co, Ni and Fe.
 10. The sensor of claim 1 wherein FL1 comprises a ferromagnetic Heusler alloy.
 11. The sensor of claim 1 wherein the net magnetic moment/area of the APF structure is less than the magnetic moment/area of 100 Å of Ni₈₀Fe₂₀.
 12. The sensor of claim 1 wherein the thickness of FL1 is at least 110% of the spin diffusion length of the material from which FL1 is formed.
 13. The sensor of claim 1 wherein FL1 is formed essentially of Ni₈₀Fe₂₀ and the thickness of FL1 is at least 45 Å.
 14. The sensor of claim 1 wherein the current-induced noise of the sensor when the sense current is applied perpendicular to the planes of the layers in the sensor in a direction from the pinned ferromagnetic layer to the APF structure is substantially less than the current-induced noise when sense current is in a direction from the APF structure to the pinned ferromagnetic layer.
 15. The sensor of claim 1 wherein the pinned ferromagnetic layer comprises an antiparallel (AP) pinned structure comprising a first AP-pinned (AP1) ferromagnetic layer having an in-plane magnetization direction, a second AP-pinned (AP2) ferromagnetic layer having an in-plane magnetization direction substantially antiparallel to the magnetization direction of the AP1 layer, and an AP coupling (APC) layer between and in contact with the AP1 and AP2 layers; and wherein the electrically conductive spacer layer is on the AP2 layer.
 16. The sensor of claim 15 wherein the AP-pinned structure is a self-pinned structure.
 17. The sensor of claim 15 further comprising an antiferromagnetic layer exchange-coupled to the AP1 layer for pinning the magnetization direction of the AP1 layer.
 18. The sensor of claim 15 further comprising a hard magnetic layer in contact with the AP1 layer for pinning the magnetization direction of the AP1 layer.
 19. The sensor of claim 1 wherein the sensor is a magnetoresistive read head for reading magnetically recorded data from tracks on a magnetic recording medium, and wherein the electrical leads are shields formed of magnetically permeable material.
 20. A current-perpendicular-to-the-plane magnetoresistive read head for reading magnetically recorded data from tracks on a magnetic recording medium when a sense current is applied to the head in a direction from a first electrical lead to a second electrical lead, the head comprising: a substrate; a first electrical lead layer on the substrate; one and only one pinned ferromagnetic layer, said one and only one pinned ferromagnetic layer being located on the first electrical lead layer and having an in-plane magnetization direction; an electrically conductive spacer layer on the pinned ferromagnetic layer and consisting essentially of an element selected from the group consisting of Cu, Ag and Au; an antiparallel free (APF) structure comprising (a) a first free ferromagnetic layer (FL1) on the spacer layer and having an in-plane magnetization direction, (b) a second free ferromagnetic layer (FL2) having an in-plane magnetization direction substantially antiparallel to the magnetization direction of FL1 and a magnetic moment less than that of FL1, wherein each of FL1 and FL2 is formed of an alloy comprising Fe and one or both of Ni and Co and FL2 has a thickness equal to or greater than a thickness that results in a FL2 magnetic moment equivalent to 10 Å Ni₈₀Fe₂₀, and (c) an AP coupling (APC) layer between and in contact with FL1 and FL2 and consisting essentially of a material selected from the group consisting of Ru, Ir, Rh and alloys thereof; a second electrical lead layer on FL2; a ferromagnetic biasing layer for biasing the magnetization of the APF structure, the biasing layer having a magnetization direction substantially antiparallel to the magnetization direction of FL2 in the absence of an external magnetic field; and a source of sense current connected to the first and second electrical lead layers and supplying current to the sensor in a direction from the first electrical lead layer through said one and only one pinned ferromagnetic layer, spacer layer, and APF structure to the second electrical lead layer.
 21. The head of claim 20 wherein FL2 has a thickness equal to or greater than a thickness that results in a FL2 magnetic moment equivalent to 15 Å Ni₈₀Fe₂₀.
 22. The head of claim 20 wherein FL2 has a thickness equal to or greater than a thickness that results in a FL2 magnetic moment equivalent to 10 Å Ni₈₀Fe₂₀ and equal to or less than a thickness that results in a FL2 magnetic moment equivalent to 15 Å Ni₈₀Fe₂₀.
 23. The head of claim 20 wherein the biasing layer is a hard magnetic layer located near the edges of FL1.
 24. The head of claim 20 wherein the net magnetic moment/area of the APF structure is less than the magnetic moment/area of 100 Å of Ni₈₀Fe₂₀.
 25. The head of claim 20 wherein the thickness of FL1 is at least 110% of the spin diffusion length of the material from which FL1 is formed.
 26. The head of claim 20 wherein the current-induced noise of the head when sense current is applied in a direction from said first electrical lead layer to said second electrical lead layer is substantially less than the current-induced noise when sense current is applied in a direction from said second electrical lead layer to said first electrical lead layer.
 27. The head of claim 20 wherein the pinned ferromagnetic layer comprises an antiparallel (AP) pinned structure comprising a first AP-pinned (AP1) ferromagnetic layer having an in-plane magnetization direction, a second AP-pinned (AP2) ferromagnetic layer having an in-plane magnetization direction substantially antiparallel to the magnetization direction of the AP1 layer, and an AP coupling (APC) layer between and in contact with the AP1 and AP2 layers; and wherein the electrically conductive spacer layer is on the AP2 layer. 